Achieving the lowest noise in a signal chain is vital as industry trends push the boundaries of resolution and precision. And when pushing these boundaries, it’s important to consider not just the noise of signal-chain components such as analog-to-digital converters (ADCs) and amplifiers, but also power products such as switching and low-dropout regulators (LDOs). Advances in silicon technologies have reduced the trade-offs when attempting to achieve low noise and high precision in power topologies.

Recent trends in 24-bit delta-sigma ADCs have increased sampling speeds and lowered power consumption. New low-noise power supplies and low-noise voltage references can take advantage of these trends and help ADCs achieve high-resolution measurements in low-power applications.

To achieve the lowest noise, let’s review the sources of noise in the signal chain and power architecture. Figure 1 shows a typical signal-chain application centered around an ADC that requires an external voltage reference, clock and signal-conditioning circuit. Every component in Figure 1 contributes to system noise and requires optimization.

Figure 1 Common signal-chain power architecture.

Noise in an ADC can cause errors in precise voltage measurements. You must consider the total contribution of noise in the signal chain from internal and external sources. Total noise is often a combination of ADC thermal noise, ADC-quantization noise, amplifier noise, voltage-reference noise and power-supply noise.

Equation 1 depicts the total referred noise at the input of the ADC (at full-scale voltage) as it measures the sensor based on Figure 1. The main design challenge is to optimize all noise sources to achieve the noise target that the application requires. In Equation 1, the ADC’s power-supply rejection ratio (PSRR) reduces the power-supply noise, which is plotted out to 1 MHz:

Given the existence of uncorrelated noise sources, the total noise is the root sum square of all sources, which heavily favors the largest noise source. One noisy component can heavily skew the measurement. For example, if a voltage reference contributes more noise than an ADC and power supply, reducing noise on the voltage reference will be the best way to lower the system noise, as shown in Figure 2 and Figure 3. In addition, ADC noise types vary with resolution: quantization noise is significant for a 16-bit ADC, but you can ignore it for a 24-bit ADC.

Figure 2 Voltage reference dominant noise.

Figure 3 ADC thermal dominant noise.

Power-supply noise is random and occurs in all semiconductor power devices and power topologies. The focus of this white paper is signals below 100 kHz, as signals above this are often attributable to switching ripple or electromagnetic interference (EMI). You can also further separate noise into low-frequency noise (0.1 Hz to 10 Hz) and high-frequency noise (100 Hz to 100 kHz), with differing requirements and design challenges, as shown in Figure 4.

Figure 4 Noise-frequency spectrum.

Low-frequency noise is often specified as the peak-to-peak noise between 0.1 Hz and 10 Hz that a semiconductor device naturally produces given the combination of its silicon properties and design architecture. This low-frequency noise is often visible in an oscilloscope when zooming into a voltage rail at high resolution, as shown in Figure 5, and is often the cause of errors in precision DC measurements. ADC applications where low-frequency noise is a critical specification include battery measurements, energy metering, seismic measurements and even semiconductor test measurements.

Figure 5 Low-frequency noise on an oscilloscope.

The alternative is high-frequency noise, which is in the band of 100 Hz to 100 kHz and can include white noise, switching noise and clock jitter, as shown in Figure 6. High-frequency noise sources can also come from the environment, through coupling from EMI. For example, an ADC can experience errors from a noisy power supply. EMI from the same noisy power supply can lead to increased clock jitter, which if excessive can degrade signal-to-noise performance.

It is becoming increasingly important to lower high-frequency noise caused by rising clock frequencies in digital circuits, which are more susceptible to jitter. ADC applications where high-frequency noise is a critical specification include power-line quality monitors, digital signal processing applications and radio-frequency (RF) communications equipment.

Figure 6 Buck regulator switching noise.

One way to lower noise is to increase the power in the system, but there is often a power budget, making it necessary to maximize noise performance with limited power. TI’s low-noise voltage reference portfolio includes low-power options such as the REF33, REF34 and REF35 families, which push the boundary between power and noise for low-power ADCs with high precision. A lower quiescent current (I_Q) voltage reference is beneficial for portable or edge applications, such as two-wire transmitters, that have a limited power budget.

Innovations in efficient band-gap circuits and output buffers have improved the power-to-noise ratio of voltage references. The REF33, REF34 and REF35 are staple devices in TI’s voltage reference portfolio for low noise and low power. Figure 7 compares their noise and power and highlights the innovation of the REF35.

Figure 7 Voltage reference power and noise.

One common application for low noise is portable medical equipment such as portable electrocardiogram machines. The ADS124S08 family of 24-bit ADCs has power consumption as low as 280 μA to minimize power draw in field instruments and edge devices that have limited power budgets. Table 1 compares the REF35 with the internal ADS124S08 voltage reference and highlights the improved accuracy with I_Q. The REF35’s low noise and high accuracy improves both quantization and gain errors while lowering system power. The benefit of a flexible voltage reference voltage enables further optimization for maximizing the full-scale range of the ADS124S08.

The REF35 also pairs with the ADS127L11, which is an ADC focused on DC precision with low power consumption. The REF35 offers a 10x reduction in supply current compared to the REF34 which makes it a stronger pairing with the ADS127L11 in low-speed mode. This pairing enables the ADS127L11 to achieve accuracy in power quality analyzer systems that require high precision, or machine vibration systems that need low power in order to balance solution size, resolution and bandwidth.

Many types of voltage references provide ultra-low noise voltage levels. However, buried Zener voltage references stand apart as having particularly low noise. Buried Zener voltage references usually do not require gain to generate an output voltage, thus reducing the noise. Buried Zener voltage references often are used to provide a “golden” voltage level for a high precision system. The voltage reference is used for calibration or with ultra-precise data converters, such as DAC11001B, as shown in Figure 8.

Figure 8 REF80 used as the voltage reference of DAC11001B with reference buffer.

When using buried Zener devices for calibration, there are three main parameters that are important to consider: temperature drift, long-term drift, and noise. During calibration, system data converters use the stable and low-noise voltage provided by a buried Zener voltage reference, such as REF80, to determine the gain and offset error of an ADC or DAC. REF80 has an ultra-low, 0.16ppm_p-p noise specification. For accurate calibration, the voltage level cannot vary over time or temperature, and the value provided must be low noise to ensure the errors observed during calibration can be effectively compensated for.

When REF80 is used with DAC11001B, it must be buffered to allow for good dynamic performance. These buffers will add more noise to the reference circuit, and thus the overall signal chain. For this reason, low noise operation amplifiers must be used to maintain low noise. OPA828 is a low noise operational amplifier with 4nV/√Hz noise at 1kHz that is often used in the reference buffer circuit.

In order to ensure that the noise from REF80 is mostly only coming from the voltage reference, it is also important to power REF80 using low noise low dropouts (LDOs). REF80 is unique in that it has an internal heater. This heater holds the die at a consistent temperature regardless of the surrounding environment. This heater enables the low drift specifications of REF80. The heater and reference power are separate from each other. For this reason, both the heater (HEATP) and VDD (drain supply) need a power supply. The REF80 pinout is shown in Figure 9.

Figure 9 REF80 LCCC 20-pin package pinout.

The heater of REF80 typically draws up to 335mA upon startup, settling to 18mA to 75mA while VDD only typically requires 15mA of quiescent current. Additionally, the noise on the voltage output of REF80 (REF_Z) is dependent on the buried Zener circuit, not the heater. The architecture of REF80 is such that only the buried Zener reference has a major impact on the output noise while the heater has minimal impact. Figure 10 shows a simplified block diagram.

Figure 10 REF80 functional block diagram.

Therefore, for the lowest noise possible, the VDD pin, which powers the buried Zener reference, must use a low noise LDO to provide power. In the REF80 evaluation module, REF8EVM, the wide input voltage, ultralow noise LDO TPS7A49 is used for VDD. For the heater, the higher current output, but higher noise, LM317 is used. Figure 11 demonstrates a block diagram of the REF8EVM power supply configuration.

Figure 11 REF80 general application and power tree block diagram.

It is also an option to use one LDO for both VDD and the heater. If this is desired, another good option is TPSA4701, which has ultralow noise and a higher output current capability to power both VDD and HEATP.

For the highest precision of technologies and applications, buried Zener voltage references are one of the best options to consider. The low drift and low noise of buried Zener devices such as REF80 are essential where signal chain and calibration noise are a major concern.

High-resolution ADCs are more sensitive to voltage reference noise, which directly impacts voltage measurements because of their direct connection to data-conversion circuitry. Ultra-low-noise voltage references help high-resolution ADCs reach their full resolution potential. The REF70 has ultra-low 1/f noise that attaches with products that require ultra-low noise such as high-resolution ADCs or a multichannel analog front end such as the AFE2256. Adding low-pass filters on the output of the voltage reference lowers broadband noise and thus lowers system noise, as shown in Figure 12.

When designing a low-pass filter, it is important to ensure that the output impedance does not degrade AC performance. This can occur in resistor-capacitor low-pass filters where a large series resistance affects the load transients caused by output current fluctuations. Choose a low-pass filter bandwidth cutoff frequency under 10 Hz to limit the impact of broadband noise.

Figure 12 REF7025 application with an external low-pass filter.